In DC-DC buck converter applications with synchronous low-side metal oxide semiconductor field effect transistors (MOSFETs), a large voltage overshoot can occur when the high-side MOSFET turns on. This overshoot may be caused by “snappy” behavior of the low-side MOSFET's body diode and can cause voltage overshoot and excessive ringing on the node connecting the drain of the low-side MOSFET to the source of the high-side MOSFET. The voltage overshoot can exceed the voltage rating of the low-side MOSFET, leading to reliability issues, e.g., reduced performance, shortened lifetime or failure. The ringing can disturb sensitive circuitry nearby, and the noise from the ringing can also cause electromagnetic interference (EMI).
FIG. 1 illustrates an exemplary DC-DC buck converter circuit 100, in accordance with the conventional art. Buck converter circuit 100 comprises a low-side MOSFET switch 110 and a high-side MOSFET switch 120. It is to be appreciated that low-side MOSFET switch 110 comprises a body diode (not shown). The MOSFET body-diode is a side effect of the fabrication process and is generally not considered to be a “good” diode. In comparison to discrete high-speed diodes, the body-diode's reverse recovery time is very long, for example, the body diode takes a long time to turn off when the current flowing through it changes direction. This may lead to a shoot through or snap back condition when the opposing switch, e.g., high-side MOSFET switch 120, is turned on.
The high-side MOSFET switch 120 and low-side MOSFET switch 110 are configured to be controlled by control circuitry, e.g., to be turned on and off, to produce an output voltage Vout. Buck converter 100 further comprises a switching node 130, for example, the coupling of the drain of the low-side MOSFET 110 to the source of the high-side MOSFET 120. In operation, switching node 130 may be subjected to ringing noise. In addition to the numerous deleterious effects described above, the ringing may approach or exceed the voltage rating of low-side MOSFET switch 110. For example, for reliability reasons, designers may desire to operate low-side MOSFET switch 110 with a peak voltage of no more than 80% of its maximum voltage rating.
Unfortunately, substitution for low-side MOSFET switch 110 with a part of a higher voltage rating is not a desirable solution. For example, MOSFETs with higher voltage ratings tend to have greater internal resistance and greater switching loss, resulting in lower switching efficiency. Thus, the conventional art does not offer a desirable solution for these problems.